Clostridium difficile infections (CDI) have a high rate of recurrence. Recurrent CDI are more refractory to antibiotics and are associated with significantly increased mortality. Studies suggest that over half of the recurrent CDI are due to relapse with the same strain and the remainder due to reinfection with a new strain. CDI recurrence therefore draws attention to two important issues; 1) persistent colonization of C. difficile in the host gut and 2) the lack of protective immunity from previous episodes of CDI. Neither of these issues is clearly understood and this lack of understanding hinders the management and prevention of recurrent CDI. Our long-term goals are two-fold; a) to elucidate the molecular mechanism(s) underlying C. difficile persistent colonization and the lack of protection from previous episodes of CDI, and b) to use the knowledge gained to develop strategies to better prevent and treat recurrent CDI. Due to intrinsic resistance to antibiotics an host antimicrobials, C. difficile spores are key to persistent infections and disease transmission. Recent evidence from studies in C. difficile and closely related Bacillus anthracis suggests that in addition to their intrinsic resilience, there are spore-specific mechanisms that promote colonization. Based on work by us and other groups, the central hypothesis in this proposal is that surface proteins of C. difficile spores mediate binding to complement regulator factor H and that the binding promotes spore colonization of the gut and recurrent CDI. The overall objective in this proposal is to identify the specific C. difficile surface protein(s) that mediate binding t factor H and to determine the biological consequences of the binding. Two specific aims are proposed. Aim 1 will identify the specific factor H binding protein on C. difficile spores using a variety of strategies. Aim 2 will determine the biological activities of the binding using in vitro assays and in a mouse model of CDI. The effect of factor H binding on complement activation, adherence of C. difficile spores to host cells, gut colonization, disease severity and antibody responses will be determined. The concept that C. difficile spores not only passively contribute to recurrent CDI due to their intrinsic resilience, but also adopt a specific mechanism to actively promote persistent colonization and recurrent infections is novel in the C. dfficile field. Results from the proposed work may provide for the first time insights into this mechanism and inform future investigations to further dissect the molecular details of this mechanism. Further studies may lead to novel strategies to better manage and prevent recurrent CDI.